1. Field
The present disclosure relates to a screw control method of an injection molding machine, and more particularly, to a screw control method capable of controlling the advance and retraction of a screw so as to uniformly distribute the internal pressure of a melted resin, thereby reducing variations in the weight of an injection product injected from the screw.
2. Description of the Related Art
In general, an injection apparatus is an apparatus for injecting a resin into a mold manufactured to be suitable for the shape of a product to be formed, and is an apparatus that melts resin granules in a solid state using mechanical energy generated by rotation of a screw installed in a barrel and heat energy generated by a heater mounted outside the barrel, and injects the melted resin into the mold manufactured to be suitable for the shape of the product to be formed so that the resin solidifies, thereby obtaining a product with a desired shape.
The screw installed in the barrel includes a screw part for transporting and melting the solid resin granules and a screw head part for preventing backflow of the resin when the melted resin is injected into the mold.
The granular resin supplied through a hopper is inserted into the barrel mounted with the heater, and the resin is melted in the barrel by shearing heat generated by the screw as the screw connected to a hydraulic motor or an electric motor is rotated and heat generated by the heater. Thereafter, the melted resin passes through the screw head and is accumulated on the front end portion of the head.
The screw transports the resin supplied through the hopper in a screw head direction while moving backward during transportation of the melted resin, such that the resin is continuously accumulated on the front end portion of the screw head. When the melted resin is continuously accumulated on the front end portion of the screw head to have a designated amount and measurement is completed, the screw stops rotating. Thereafter, the screw advances in the axial direction with pressure and speed corresponding to forming conditions and pushes the melted resin into the mold through a discharge opening of the barrel.
If a resin congestion occurs in the screw head part of the injection device, the detention time of the resin is lengthened, and oxidation or carbonation of the resin due to decomposition of polymer may occur.
According to the related art, after completing the measurement, the melted resin is injected as the screw immediately advances while stopping rotation. However, since the melted resin accumulated on the front end portion of the screw head is accumulated as the solid resin moves forward while being melted by the rotation of the screw, the pressure in the melted resin varies depending on the position. That is, since the melted resin farther from the screw head is generated earlier than the melted resin closer to the screw head and thus receives pressure of the melted resin that advances due to the rotation of the screw, the melted resin farther from the screw head has higher internal pressure than those of other parts.
When injection products are continuously made by injecting the melted resin in which pressure is not uniformly distributed, the weights of the injection products vary, resulting in degradation of reliability of products.
Therefore, there is a demand for a method of enhancing quality reliability of injection products by uniformalizing the pressure distribution depending on the position of a melted resin generated in an injection molding machine.